1. Field of the Invention
The present invention relates to a circuit arrangement for determining input signals which exceed a given minimum duration, in particular call signals governed by a continuous signal in teleprinter exchanges, employing a delay circuit which can be triggered by a relevant input signal and a logic element which is connected by one input to the output of the delay circuit and at the other input receives the relevant input signal, which emits an output signal in the event that the input signal supply to its further input is longer in duration and corresponds to the delay time of the delay circuit.
2. Description of the Prior Art
A circuit is already known in the art for reporting the reductions in loop current, which indicate the beginning of dialing, in telephone exchanges (Austrian AS No. A 50 63/63now Austrian Pat. No. 239,321 of Mar. 25, 1965), wherein the subscriber line is connected to a register and to the base electrode of two transistors, whose parallelconnected emitter electrodes are connected to a capacitor and whose collector electrodes are connected to differently dimensioned load resistors, wherein a measuring device which corresponds to a threshold value is connected to the collector electrode of the transistor connected to the greater load resistor. Although this known circuit arrangement enables reductions in loop current, which indicate the beginning of dialing, to be determined in the relevant subscriber line and an appropriate message signal to be emitted, it does not facilitate a differentiation between beginning-of-dialing signals of this type and interference signals which can likewise lead to a reduction in loop current in the relevant subscriber line. An electronic pulse circuit for suppressing interference pulses which have an interval shorter than the specified interval, while adhering to the pulse trailing edge without delay is also known (German AS No. 1 173 938). In this known pulse circuit, one input of an AND circuit is connected directly via a line, and the second input via an integrator, which represents a delay circuit, to a sampling device. This known pulse circuit only emits an output signal when an output signal with which it has been supplied extends beyond a minimum duration, determined by the dimensioning of the integrator. In this manner, however, all the signals and pulses consecutively fed to the input of the relevant pulse circuit are evaluated. This is undesirable in cases in which it is adequate to simply evaluate the duration of a first signal occurring within a sequence of consecutive signals.